Mattress core



FIG. 1 top view showing my design for a mattress core thereof;

FIG. 2 is a first end view;

FIG. 3 is a second end view;

FIG. 4 is a bottom view;

FIG. 5 is a first side view;

FIG. 6 is a second side view; and,

FIG. 7 is top perspective view. 

The ornamental design for mattress core, as shown and described. 